1. Field of the Invention
The present invention relates to a magnetic head and a method of manufacturing thereof. More particularly, it relates to a magnetic head for use in a magnetic recording and reproducing apparatus such as a video tape recorder (VTR) or a digital audio tape recorder (DAT), and a method of manufacturing thereof.
2. Description of the Prior Art
In recent years, there has been an increasing tendency for high density recorded signals in a magnetic recording and reproducing apparatus such as a VTR or a DAT. Metal tapes of high coercive force formed by using ferromagnetic metal powder such as Fe, Co or Ni as magnetic powder have been used for such high-density recording. For example, metal tapes having a high coercive force, Hc=about 1400 to 1500 oersteds are used for small-sized VTRs known as 8 mm video tape recorders. This is because a recording medium making it possible to shorten wavelengths of recorded signals is required for the necessity of increasing a recording density to reduce a size of a magnetic recording and reproducing apparatus.
On the other hand, if a conventional magnetic head made only of ferrite is used for recording on a metal tape, a magnetic saturation phenomenon occurs because a saturation flux density of ferrite is only 5500 gauss at most, and accordingly optimum performance of the metal tape cannot be achieved. A magnetic head adapted for a metal tape having a high coercive force needs to have a high saturation flux density in the vicinity of a gap of a magnetic core, other than high-frequency characteristics and abrasion resistance of the magnetic core as generally required for magnetic heads. In order to satisfy such requirements, it is proposed to use, as a magnetic head adapted for a metal tape, a magnetic head made of a metallic magnetic material such as permalloy, sendust or amorphous magnetic material having a larger saturation magnetization than that of ferrite used for a magnetic core (such a magnetic head being hereinafter referred to as a composite type magnetic head). Such a composite type magnetic head has excellent characteristics in reliability, magnetic properties, abrasion resistance and the like.
FIG. 1 is a perspective view showing an appearance of a conventional magnetic head. As shown in FIG. 1, a pair of magnetic core halves 1a and 1b made of a ferromagnetic oxide such as Mn-Zn ferrite are opposed to each other through a non-magnetic material with a magnetic gap 2 being provided therebetween, and thin films 3a and 3b of a ferromagnetic metal such as sendust having a high saturation flux density are formed near the magnetic gap 2. The magnetic core halves 1a and 1b are joined by glass 4 with a coil groove 5 being provided.
In the composite type magnetic head thus provided, the thin films 3a and 3b of the ferromagnetic metal are deposited by sputtering on upper surface areas of the base of the ferromagnetic oxide where mirror surface finishing has been applied. However, junction regions between the thin films of the ferromagnetic metal and the base of the ferromagnetic oxide are non-magnetized due to interdiffusion and chemical reactions of constituent elements, or irregular arrangement in crystal structure. As a result, those regions function as pseudo gaps, exerting adverse effect on the performance of the magnetic head.
More specifically, as shown in FIG. 1, pseudo gaps are formed on boundary faces 6a and 6b between the magnetic core halves 1a and 1b and the thin films 3a and 3b of the ferromagnetic metal, respectively, other than the prescribed magnetic gap 2. If a singly reverse-magnetized recorded signal on a magnetic tape is reproduced by using the magnetic head having boundary faces 6a and 6b where such pseudo gaps are formed as shown in FIG. 3, pseudo signals 8a and 8b are reproduced with deviations of time .tau.=t/v before and after reproduction of the prescribed signal 7. In the equation .tau.=t/v, t represents a thickness of each of the ferromagnetic metal thin films 3a and 3b in the relative traveling direction defined between the head and the tape, and v represents a relative traveling speed defined between the head and the tape.
On the other hand, if continuously reverse-magnetized recorded signals having a recorded wavelength .lambda. substantially equal to or shorter than t are reproduced, the pseudo signals 8a and 8b as shown in FIG. 3 cannot be readily observed. However, if the frequency characteristics of the reproduced output are measured, a frequency characteristic curve which waves with peaks at a frequency satisfying f=n.multidot.(v/t) and bottoms at a frequency satisfying f=(n-1/2).multidot.(v/t), where f indicates frequency (v/.lambda.) and n indicates a natural number, is obtained as shown in FIG. 4 by superposition of the reproduced output through the pseudo gap onto the reproduced output through the prescribed magnetic gap 2. Accordingly, if a magnetic head having boundary faces 6a and 6b where such pseudo gaps are formed is used for a VTR or DAT, pseudo signals due to the pseudo gaps cause noises, resulting in a deterioration of the quality of the picture or adverse effects on the performance of the magnetic head, such as increase of an error rate. Particularly, in the case of using a magnetic head having a magnetic gap 2 parallel to the boundary faces 6a and 6b where such pseudo gaps as shown in FIG. 1 are generated, a waving phenomenon in the frequency characteristics of the reproduced output is observed and the S/N ratio is deteriorated.
In order to suppress generation of such pseudo gaps, it is proposed to adopt a method for applying reverse sputtering to a surface of a base member of a ferromagnetic oxide such as ferrite under suitable power supply conditions immediately before formation of thin films of ferromagnetic metal, as disclosed for example in Japanese Patent Laying-Open No. 57115/1987. However, even this method cannot completely remove portions adversely affected by the treating process of the thin films, which would cause the pseudo gaps, and accordingly this method is not sufficiently effective for preventing generation of pseudo gaps.
Further, in order to suppress generation of pseudo gaps, another proposal is made in Japanese Patent Laying-Open No. 145510/1987, which discloses a magnetic head having, near a magnetic gap, a reaction prevention layer of an oxide material and a metal material in the interface between a ferromagnetic oxide and each thin film of ferromagnetic metal. However, generation of pseudo gaps cannot be sufficiently suppressed only by such interposition of the reaction prevention layer on the surface forming a gap.
Further, as shown in FIG. 2, a composite type magnetic head is proposed in which boundary faces 6a and 6b between magnetic core halves 1a and 1b and thin films 3a and 3b of ferromagnetic metal, respectively, have an inclination and are not parallel to the faces forming the magnetic gap 2, causing no adverse effect to the performance of the head even if the pseudo gaps are generated. However, the magnetic head having such structure is not suited for mass production because the manufacturing process thereof is complicated and the manufacturing cost comes to be high compared with the magnetic head having the boundary faces 6a and 6b parallel to the magnetic gap 2 as shown in FIG. 1.